Thermofusible, or "hot-melt", adhesives are solid at room temperature. It suffices to melt them by heating between 150 and 200.degree. C., setting occurring by cooling.
The main constituents of a thermofusible adhesive are: a base copolymer, a tackifying resin, waxes, stabilizers, and optionally fillers.
Polyolefins, copolymers thereof, polyamides, and so on, may be used as the base copolymer. The base copolymer constitutes the core of the hot melt. It provides and determines its general properties, in particular, cohesion, thermal behavior, and stability.
The tackifying resin imparts better hot adhesive power to the thermofusible adhesive, and thus better adhesion at the application temperatures.
The waxes make it possible to control the viscosity and to modify the rheological curve of the base polymer as a function of the temperature.
The stabilizers are antioxidants or anti-UV agents. They increase the resistance to ageing of the mixture.
The plasticizers modify the cold properties of the hot-melt and in particular the flexibility of the adhesively bonded joint.
U.S. Pat. No. 4,816,306 of the prior art describes thermofusible adhesives composed of:
a. 35 to 45% by weight of the ethylene/n-butyl acrylate copolymer; PA1 b. 35 to 55% of a tackifying resin; PA1 c. 10 to 20% of a wax with a high melting point. PA1 A 100 parts of polymers: PA1 Aa 25 to 75% of an ethylene copolymer, PA1 Ab 75 to 25% of an ethylene/unsaturated carboxylic acid ester copolymer, and PA1 B 25 to 200 parts of a tackifying resin; PA1 a at least one essentially amorphous poly(alpha-olefin), PA1 b at least one copolymer of ethylene and an alkyl (meth)acrylate, PA1 viscosity at 190.degree. C. between 3000 and 50,000 mPa s, PA1 softening temperature (bead and ring, ASTM D 28) between 80 and 160.degree. C., PA1 needle penetration (0.1 mm) of 4 to 21, PA1 density at 23.degree. C. between 0.85 and 0.9 g/cm.sup.3, PA1 viscosity index (J) between 40 and 95 cm.sup.3 /g, PA1 molecular weight MW between 30,000 and 75,000, PA1 intrinsic viscosity (100 ml/g) between 0.4 and 0.9.
Application JP 60 096676 KOKAI published on May 30, 1985 describes thermofusible adhesives consisting of:
at least one of the polymers A being grafted with an unsaturated carboxylic acid or a derivative and the viscosity of these adhesives being, at 180.degree. C., less than 50 Pascal seconds.
The examples show that Aa is chosen from ethylene-propylene or ethylene-butene rubbers and Ab is an ethylene/ethyl acrylate copolymer, at least one being grafted with maleic anhydride.
The quality of the adhesion obtained is measured by the SAFT test according to ASTM D 4498, which measures the cohesion limit temperature at shear, and by the T peeling test according to ASTM 1876-72.